1. Field of the Invention
The present invention concerns an ultrasound converter, and in particular a so-called axial oscillator, which has a piezoelectric element that is arranged between at least two tensioning segments and at least one tensioning member for pretensioning the piezoelectric element with the tensioning segments.
2. Description of the Prior Art
Ultrasound converters are also referred to as axial oscillators or xcex/2 oscillators. The piezoelectric element has electrode surfaces which are polarized and electrically connected so that the entire structure resonates in an axial direction when an alternating current of suitable frequency is applied. The piezo element arranged between the tensioning segments is in a state of maximum mechanical tension and minimal mechanical oscillating speed. Ultrasound is emitted from the free face of one of the tensioning segments that functions as a wave guide or transmitter. To attain the highest possible wave emission with an adequate stability of the apparatus, the piezoelectric element is pretensioned with the tensioning member that acts via the tensioning segments.
It is an object of the present invention to provide an ultrasound converter as previously described which is as small as possible and which achieves the highest possible pretensioning of the piezoelectric element without adversely affecting the oscillating characteristics of the ultrasound converter.
The object is achieved with various features. A ring-shaped tension spring is used as the tensioning member which has at least one peripheral recess in its wall.
The peripheral recess in the ring-shaped tensioning member achieves a maximum spring travel while minimizing its axial length. The spring travel is large relative to the length of the device, which permits high prestressing of the piezoelectric element without exceeding the permissible load on the tension spring.
The tension spring can therefore oscillate even when highly pretensioned without adversely affecting the operation of the ultrasound converter.
Because of the small axial length of the tension spring of the invention, the spring cooperates only with those portions of the tensioning segments in the immediate vicinity of the piezoelectric element. In these areas, the oscillation amplitudes and oscillation speeds are small.
The tension spring of the present invention and the two tensioning segments are preferably threadably connected. The piezoelectric element is placed between the tensioning members and is threaded into the ring-shaped tensioning spring. The minimal axial length of the converter of the present invention prevents that the tensioning spring and the tensioning segments cooperate at portions of the threads which are axially far from the piezoelectric element where strong oscillations activities are encountered. An undesirable dampening in the areas of the threads is thereby avoided.
A further advantage of the present invention is that the dimensions of the peripheral recess of the tensioning spring can be adapted to the particular requirements of any given ultrasound converter. The peripheral recess is preferably dimensioned so that, for any given desired pretensioning of the piezoelectric element, the spring travel is sufficiently large without exceeding the maximum permitted tension in or loading of the tension spring material.
It is further possible to dimension the peripheral recess, including especially also as a function of the dimensions of the tension spring itself, so the operation of converters as broad band axial oscillators is not adversely affected by natural oscillation frequency modes of the tension spring at the operating frequency range or the principal transmission frequency range. It is further possible to configure the peripheral recess of the tension spring so that natural oscillation frequency modes are avoided which may be outside the operating frequency of the converter but which nevertheless can lead to undesirable interference in the operation of the converter. In this manner, the present invention makes it possible to optimize the natural oscillation frequency spectrum of the tension spring in dependence on the design and operating characteristics of the ultrasound converter.
The large spring travel that is attainable with the present invention has the further advantage that the influence of environmental parameters, such as temperature, on the operation of the ultrasound converter is minimal, and aging of the spring material can be compensated for. Accordingly, with the present invention, adjustments can be made to account for variations in the operations and/or environmental conditions and aging.
The peripheral recess is preferably shaped as a slit or gap. The circumferential recess further preferably extends in a radial direction through the entire wall of the ring spring; that is, the wall of the ring spring is preferably interrupted in an axial direction over the angle over which the peripheral recess extends.
In a particularly preferred embodiment of the present invention, an oscillation dampening material at least partially fills the peripheral recess. Interfering start and ending oscillations can in this manner be dampened and rendered harmless.
The use of a material that can be cast into the peripheral recesses is preferred.
The use of a dampening mass which at least partially fills the peripheral recess is particularly advantageous for converters which generate or receive short ultrasound impulses because for such converters the beginning and ending oscillations have a relatively large impact on measurements.
According to another preferred embodiment, a good way to adapt the tensioning spring to any given application is to provide two axially spaced peripheral recesses in the tensioning spring.
Such a two-stage construction combines high tensioning with large spring travel without appreciably increasing the axial length of the device and without significantly making the elimination of interfering natural oscillation frequency modes more difficult.
The object of the present invention can also be satisfactorily attained by providing three or more peripheral recesses. Eliminating or reducing interfering natural oscillation frequency modes in this manner however is more costly. In such instances, it is possible, and the present invention permits, to configure the components of the ultrasound converter to match the optimal characteristics of the tensioning spring instead of designing the tensioning spring to fit the ultrasound converter.